triangulate faces obj format problem
In the Wavefront file-format, the second number for every element for a face is the texture-coordinate

The quick way to get it running:
Select your object, go into edit-mode, press U and select Unwrap.


When exporting, make sure you have Include UVs selected.



Outcome (only faces): 

f 2/1/1 3/2/1 4/3/1
f 8/1/2 7/2/2 6/3/2
f 5/1/3 6/2/3 2/3/3
f 6/1/4 7/2/4 3/3/4
f 3/4/5 7/1/5 8/2/5
f 1/1/6 4/2/6 8/3/6
f 1/4/1 2/1/1 4/3/1
f 5/4/2 8/1/2 6/3/2
f 1/4/3 5/1/3 2/3/3
f 2/4/4 6/1/4 3/3/4
f 4/3/5 3/4/5 8/2/5
f 5/4/6 1/1/6 8/3/6